Ground vs Satellite vs Airborne Gravity Surveys: Which One Should You Actually Pay For?

Last month a client in Chagai called me at 11pm. He'd just been quoted PKR 4.2 crore for an airborne gravity survey over a 180 sq km copper prospect. His question was simple: is this the right survey, or am I being sold something I don't need?

Honestly, that's the question most mine owners in Pakistan should be asking before they sign anything.

Gravity surveys measure tiny changes in Earth's gravitational pull. Denser rocks (think chromite, massive sulphides, iron) pull harder. Lighter rocks (granite intrusions, salt domes, sediments) pull less. That difference — measured in milligals — is what tells you where the ore body might be hiding underground.

But there are three ways to collect that data. And they're not interchangeable. I've paid for all three across my own 15 mines in Gilgit Baltistan, and I've watched clients burn money picking the wrong one.

Satellite Gravity: Cheap, Fast, and Where Every Project Should Start

Satellite gravity data comes from missions like GRACE and GOCE. The resolution is coarse — we're talking pixels around 5 to 10 km on the ground for raw data, though modern processing gets us to about 2 km for regional interpretation.

That sounds terrible. And for finding a specific ore body, it is.

But here's the thing. Before you spend a single rupee on ground crews or aircraft, you need to know if your license area even sits on the right kind of crustal structure. Satellite gravity data mining tells you that. It shows deep basement highs, major fault systems, ophiolite belts, sedimentary basin edges — the big regional stuff.

At GeoMine AI, we fold satellite gravity into our breeze geo mineral analysis alongside Sentinel-2, ASTER, and SRTM DEM. A client sends us a license polygon. We come back with a report that shows whether the regional gravity signature even supports the target mineral they're chasing. Cost to the client: a fraction of what a physical survey runs.

For a 100 sq km area, satellite-based regional analysis through geomines runs you a few lakhs at most. An airborne survey over the same footprint? Two to five crore, easy.

So satellite is the filter. It's not the answer.

Airborne Gravity: The Middle Ground Nobody Explains Properly

Airborne gravity — usually flown on a fixed-wing aircraft or helicopter with a gravimeter and GPS — gives you resolution of roughly 200 to 500 meters between flight lines. Much better than satellite. Much faster than ground.

When does it make sense? Big license areas. Rough terrain where ground crews can't walk. Projects where you already know (from satellite work) that something's there, but you don't know exactly where.

I'd say airborne makes sense for anything above 150 sq km in mountainous zones like Chitral, Kohistan, or northern Balochistan. Below that, the mobilization costs kill the economics. You're paying to fly the aircraft in, calibrate, and fly it out — whether your survey is 50 km or 500.

One thing people don't tell you: airborne gravity in Pakistan requires clearances that can take 3 to 7 months depending on the province and proximity to sensitive zones. I got this wrong on my first project in GB — assumed I could book a survey and fly within 6 weeks. Reality was closer to 5 months.

Accuracy-wise, modern airborne gravimeters hit around 0.6 to 1.0 milligal. Good enough to find most economic ore bodies deeper than about 100 meters. Not good enough for near-surface small targets.

Ground Gravity: The Slow, Expensive Truth-Teller

Ground gravity survey is what geologists have been doing since the 1930s. A crew walks the terrain with a gravimeter (Scintrex CG-6 is the industry standard now), takes readings at station spacings of 25 to 200 meters, ties everything to a local base station, and applies corrections for elevation, latitude, and terrain.

Accuracy? Around 0.005 to 0.01 milligal. That's 100x better than airborne.

The catch — it's slow and it's expensive per square km. A team of 4 people covers maybe 2 to 4 sq km per day in flat terrain. In the mountains of GB or Kohistan? Half a sq km on a good day. I've had crews take 3 weeks to cover what an aircraft would have flown in 4 hours.

But for a specific target — say, you've narrowed down to a 5 sq km zone where satellite showed a gravity high and ASTER showed hydrothermal alteration — ground gravity is the survey that tells you exactly where to drill. Not roughly. Exactly.

Cost in Pakistan runs PKR 40,000 to 90,000 per sq km depending on terrain and access. Add mobilization, camp, security in certain districts.

So Which One Should You Actually Pay For?

Here's how I think about it for every geo mining project that lands on my desk:

Start with satellite. Always. Never skip this. It costs you almost nothing relative to the answers it gives. If the regional signature is wrong, you save yourself crores. This is where a proper geomine report earns its keep before any field crew shows up.

Fly airborne if: your area is bigger than 150 sq km, terrain is rough, you have time for clearances, and satellite work already flagged something interesting but you can't pinpoint it.

Walk ground gravity if: you've narrowed to under 20 sq km, you're preparing to drill, and you need to place holes precisely. Or if your target is shallow (under 100m) and small — because that's where airborne loses resolution and ground wins.

My Chagai client from the opening? We ran satellite gravity + ASTER through GeoMine AI first. Found that his best signature sat in a 12 sq km sub-zone in the southwest of his license. He skipped the 4.2 crore airborne survey entirely and went straight to ground gravity over that smaller area for around 8 lakh. Drilled 4 holes six months later. Hit copper mineralization in two of them.

Sometimes the right survey method isn't the fanciest one. It's the one that answers the question you actually have.

What's the target mineral and license size you're working with? That's honestly the first thing I'd want to know before recommending anything.